Long Term Evolution (LTE) is rapidly emerging as the world's most dominant 4G technology, taking mobile broadband to unprecedented performance levels. To meet expectations and predictions for even higher data rates and traffic capacity—beyond what is available in current LTE networks—a densified network 199 (see FIG. 1) is needed. In scenarios where users are highly clustered, using multiple, low-output power base stations (e.g., base stations 110, 120, and 130) to complement a macro base station 100 providing basic coverage is an attractive solution—as illustrated in FIG. 1. This strategy results in a heterogeneous network (HetNet) deployment with two base station layers. The principle can be extended to more than two layers.
Traditionally, a communication device (e.g., a mobile phone or other communication device) connects to the base station (a.k.a., “node”) from which the downlink signal strength is the strongest. For example, in FIG. 1, cell 121 is the area in which the signal from the low-power node 120 (e.g., pico node) serving the cell is the strongest. Communication devices located in this cell (e.g., device 125) connect to the corresponding low-power node (i.e., node 121). In contrast, device 105 connects to node 100 as device 105 detects the strongest signal as coming from node 100.
Due to the difference in transmission power between pico nodes 110, 120, 130 and the overlying macro node 100, this strategy of having a communication device connect to the node associated with the highest power received signal does not necessarily result in the terminal connecting to the node to which it has the lowest path loss. It is, therefore, not the best node-selection strategy for achieving high uplink data rates. For example, it may be the case that the path loss from device 105 to node 120 is less than the path loss from device 105 to node 100.
The uptake of a low-power node can be expanded without increasing the output power of the node by adding an offset to the received downlink signal strength in the cell-selection process. That is, by introducing this offset, a device can be configured to connect to a low-power node even if the power of the received signal transmitted by the low-power node is less than the power of the received signal transmitted by the macro node. Increasing the uptake area of a node is sometimes referred to as range expansion. An example of range expansion is illustrated in FIG. 2. As shown by FIG. 2, in particular embodiments, range expansion may result in a device connecting to a low-power node while operating in an area (referred to herein as an “expansion zone”) despite the fact that the power of the received signal transmitted by the macro node is greater than the power of the received signal transmitted by the low-power node in this area. In some embodiments, this expansion zone may represent a predetermined geographic area. In other embodiments, the expansion zone may represent a potentially time-varying area in which a particular condition related to the radio channel(s) used by the device is satisfied. For example, in particular embodiments, the expansion zone may represent the area in which the power of the received signal transmitted by the macro node does not exceed the power of the received signal transmitted by the lower-power node by more than a predetermined offset.
The advantages of range expansion include: enhanced uplink data rates; increased capacity—receiving downlink traffic from the low-power node even if the received signal strength from the macro is higher allows for the reuse of transmission resources across low-power nodes; and improved robustness—enlarging the coverage area of a low-power node can reduce its sensitivity to ideal placement in a traffic hotspot.
A heterogeneous deployment, with a modest range expansion somewhere in the region of 3-4 dB, is already possible in the first release of LTE, Rel-8. The 3rd generation partnership project (3GPP) has recently discussed the applicability of range expansion with cell-selection offsets up to 9 dB. Such deployments are particularly problematic, as a terminal in the range-expansion zone (the striped area shown in FIG. 2) may experience very low downlink signal-to-interference ratio due to the significant difference in output power of the nodes. Specifically, downlink control signalling in the range expansion zone—which is essential for the low-power node to control transmission activity—poses a problem. Transmission of the data part is less challenging as Rel-8 supports methods for ensuring non-overlapping transmissions in the frequency domain from the macro and the low-power node using inter-cell interference coordination (ICIC).
One way to overcome the problem associated with downlink control signalling in the range expansion zone is to intelligently allocate resources. For example, by restricting macro node transmissions from using the same time and/or frequency resources as the low-power node, control signalling from the low-power node to the terminal can be protected.
Resource partitioning can be implemented in the frequency domain by using support for carrier aggregation (Rel-10) and can be implemented in the time domain, by relying on almost blank subframes (ABSs), a feature that will be fully supported in LTE Rel-11 (see FIG. 3).
Frequency-domain partitioning protects downlink control-signalling from the low-power node in the range-expansion zone by placing control signalling from the macro and low-power nodes on separate carriers.
Time-domain partitioning protects the downlink control signalling from the low-power node by reducing (or eliminating) macro transmission activity in certain subframes—which is illustrated in FIG. 3 and referred to as ABS. The low-power node is provided with data about the set of ABSs and can use this information when scheduling users who are in the range-expansion zone.
For backward compatibility, the macro node must transmit certain signals, most notably cell-specific reference signals (CRSs) and synchronization signals (PSSs/SSSs), in downlink subframes in the same way as in Rel-8. Thus, in some instances, the ABSs are, as a result, not completely blank—but they are almost blank. Terminals need to apply interference suppression to receive control signalling from the low-power node. Time-domain partitioning can thus be viewed as a terminal-centric approach to achieving excessive range expansion.
While resource partitioning reduces or solves some problems with introducing range expansion in an HetNet, other problems still exist. For example, measurement results and simulations have shown that communication devices connected to a low-power node while in the range expansion zone experience an increase in Radio Link Failures.